1. Field of the Invention
The present disclosure relates to an image forming apparatus, such as copying machines, printers, and facsimiles employing the electrophotographic or electrostatic recording process.
2. Description of the Related Art
With the improved operating speed, functionality, and colorization of electrophotographic image forming apparatuses, diverse types of printers and copying machines have been placed on the market.
With the increase in operating speed of image forming apparatuses, the life of various parts has been prolonged. For example, as a general method for prolonging the life of an image forming unit employing the contact development process, a photosensitive drum is normally separated from a developing roller and contacts the developing roller only at the time of image formation. This configuration can reduce the time of contact between the photosensitive drum and the developing roller, thus degradation of both members is restrained and the life of the image forming unit is prolonged.
In addition, various measures are taken for image quality stabilization. In particular, various optical adjustments are applied to exposure devices. For example, a laser beam printer using a laser beam as an exposure device has a problem of the stabilization of the laser beam amount. Accordingly, a certain optical adjustment (hereinafter referred to as automatic power control (APC)) is generally performed. More specifically, a part of a laser beam is detected by a light-receiving element, and an amount of driving current flowing into a laser diode is changed based on the result of detection to control the laser beam amount. Performing APC before image formation enables the exposure device to constantly maintain a constant laser beam amount, thus a formed image can be prevented from becoming unstable by environmental change, laser variation, and degradation.
It is necessary to perform APC during laser emission. This means that an electrostatic latent image is formed on the photosensitive drum during execution of the APC. Therefore, if the developing roller contacts the photosensitive drum during execution of the APC, toner may be uselessly consumed (developed). For this reason, conventionally, the developing roller contacts the photosensitive drum after completion of APC and extinction of the laser, thus preventing wasteful toner consumption and stain on the rear surface of paper.
Japanese Patent Application Laid-Open No. 03-238477 discusses an image forming apparatus which is not provided with a mechanism for separating the developing roller from the photosensitive drum. Therefore, since the developing roller constantly contacts the photosensitive drum, during execution of the APC, a developing bias different from that at the time of image formation is set to prevent wasteful toner consumption.
In recent years, to reduce user stress, a time period since the image forming apparatus receives image data until an image is printed (hereinafter referred to as first print out time (FPOT)) has been thought as important. Since shortening FPOT reduces the turnover time of various parts, shortening FPOT has become an important subject also from the viewpoint of life extension. Therefore, the conventional configuration in which the developing roller contacts the photosensitive drum after completion of APC is not desirable because of an increase in FPOT.
With the configuration discussed in Japanese Patent Application Laid-Open No. 03-238477, toner may be consumed even if the developing bias during execution of the APC is controlled so that toner is not consumed. The reason will be described below. Generally, the photosensitive drum sensitivity varies, for example, according to individual differences and degradation thereof. In other words, irradiation of an identical laser beam amount does not necessarily provide the same potential on the photosensitive drum. Such variation in the photosensitive drum sensitivity may produce a potential difference between the photosensitive drum and the developing roller causing toner movement therebetween, possibly resulting in wasteful toner consumption.